The prior art method of implementing loopback of repeaters has been used by the Bell System for some time. In the prior art system, a phase modulation has been used to generate tone bursts and hardware is embedded in the data clock regeneration circuitry to detect these bursts. When using a phase modulation of the data signal, there is a resultant skewing of the output data and clock signals. At the prior art data frequency of 44 megabytes, the skewing of the output data lies within a reasonable range and the hardware design and additional analog circuitry is within a cost range that it is still somewhat cost effective. At frequencies of over 100 megabytes, the skew of the data requires special temperature compensation for the repeater circuitry and a closer tolerance on all clock generation specifications. Further, the analog circuitry needed to do the primary job of detection and to provide the clock extraction techniques becomes much more complex. Accordingly, the cost of such circuitry requires a further look at how the system can best be placed in a loopback condition for testing purposes.
Since the data being transmitted during test conditions is by definition not reliable and since loopback conditions in a repeater station between end terminals is going to disrupt some of the data anyhow, the present concept specifically interrupts all transmission of data by transmitting a null signal from one end terminal towards the other wherein each series connected repeater in succession is reinitialized to a startup condition. After sufficient time has passed for the repeaters to complete their reinitialization, a set of N+1 tone bursts are transmitted through the communication link from the end terminal. Each of the repeater stations prior to the Nth repeater station upon receiving more than two of the tone bursts of a frequency other than the data frequency signal will respond by forwarding to the next repeater in the series link one less pulse than it received. When a repeater station receives only two pulses of the test frequency tone burst, it reconfigures itself into a loopback condition and returns a null signal back to the sending end terminal. After an appropriate amount of time, a single pulse of the test frequency is sent back to the end terminal as an acknowledgement that the signal has been received and a loopback condition is in existence. The end terminal can now transmit actual data and test the returning data for identity for as long as is necessary to complete the test operation. If the test proves satisfactory, a new null signal is supplied to the communication link and one more pulse is added to the set of pulses than was previously transmitted and the next repeater station in line is set to a loopback condition. This process is repeated until a test fails thereby indicating the problem area in the communication link. Since repeater stations in present day communication links are in the neighborhood of 40 miles apart, the present testing eliminates a considerable amount of traveling that would have been involved in going to each repeater station to check operation.
It is therefore an object of the present invention to provide an improved communication link testing apparatus.